The overall direction of the Molecular Mechanisms of Tumor Promotion Section is to understand the mechanisms underlying the initial events in tumor promotion. Particular emphasis is being devoted to the factors responsible for heterogeneity in the patterns of response to tumor promoters and their analogs. The isozymes of protein kinase C (PKC) are being expressed in a baculovirus system, and differ- ences in activation and response are being characterized. Studies with PKC alpha indicate that phorbol esters possess two separate mechanisms for activation with different structure-activity relations, demonstrating that heterogeneity exists at the level of the individual isozyme. Synthetic activators of PKC are being used to probe structure activity relations. Sterically constrained diacylglycerols have achieved potencies comparable to that of phorbol 12,13-diacetate. Analysis of ingenols suggests that the typical compounds require metabolism to be active. The 12-deoxyphorbol 13- monoesters have been shown to function as potent partial antagonists of PKC, dramatically blocking tumor promotion by phorbol esters among other responses. Efforts to identify the endogenous analog for the phorbol related diterpene resiniferatoxin, which interacts with a unique receptor, have revealed a serum binding protein; binding analysis of the resiniferatoxin receptor reveals cooperativity between subunits and time dependent changes in the binding characteristics, perhaps related to desensitization. A single treatment with resiniferatoxin causes long- lasting inhibition of allodynia in a rat neuropathy model. In a lung carcinoma cell line, N-chimaerin, a GAP protein with a phorbol ester binding domain, is expressed at high level, suggesting that PKC may not be the sole phorbol ester receptor in this cell line.